Steam turbines are known in which the in-flow of live steam is controlled by nozzle regulation by means of a control stage, also called the first turbine stage. Such a control stage exhibits, for example, admission sectors of varying sizes, to which the live steam is fed, in each case, by way of a live steam feed in-flow with several control valves. As a safety measure, a quick-acting stop valve is arranged in series prior the control valves. Such an arrangement is disclosed, for example, in the sales documentation of ABB Power Generation, Description No. HTGD N 12 018.
These steam turbines are typically operated at a live steam pressure that is fixedly set by the steam generator's operating parameters for all of the steam turbines' operating loads. As a result of various settings of the three or four control valves, the steam turbine can be operated at a plurality of partial load points, as well as within the load ranges pertaining to these partial load points. For this purpose, the control valves, which may be activated sequentially, are either closed or fully opened, or opened in a controlled manner.
Additional known steam turbines are operated without a control stage. The latter typically exhibit one or two live steam inlets, with a stop valve and a control valve arranged in series, in each case. Such steam turbines are disclosed, for example, in the sales documentation of ABB Power Generation, Description No. HTGD 666 159, and a valve arrangement provided therein for controlling the live steam in-flow in the same sales documentation, Description No. GMDT N06 014. The live steam pressure in these steam turbines can be variable, such as, for example, in the case of steam turbine facilities for variable pressure operation, or in the case of steam turbine facilities with a circuit combined with that of a gas turbine facility. In the case of newer steam turbine facilities, however, the live steam pressure can also be set to one single pressure level for all operating loads.
The valves in the aforementioned steam turbine facilities are preferably so configured that valve oscillations due to increased stress are kept within limits, and an operational valve life that is as long as possible and devoid of harm, is afforded.
In the steam turbines without a control stage, and particularly among those that are operated at a fixed live steam pressure, the valves must be in constantly throttled operation in order to render a safe partial load operation of the steam turbine possible. Consequently, the valves are exposed to an elevated stress in comparison with the steam turbines with a control step. Among steam turbines without a control step, the pressure is reduced by way of the valves exclusively, whereas in the case of steam turbines with a control stage, the pressure is reduced by way of the valve and the nozzles arranged in series prior to the valves. The stop valves assure safety for the live steam in-flow, but they cannot assume any throttling function. If a steam turbine is operated at fixed pressure, elevated stresses and critical pressure conditions arise in the control valves, which cause correspondingly elevated valve oscillations and an elevated risk of damage. This is the case, in particular, in steam turbines without a control stage and throttled operation in partial load operation.